Stripping in fluid coking



June 17, 1958 J. F.l MosER, .n

John Frederick Moser, Jr. Inventor By E n Attorney t1tlniteti gratos Fatemi O STRHPPENG N FLUID CKING John Frederick Moser, Jr., Baton Rouge, La., assignor to Esso Research and Engineering Company, a corporation of Delaware Application November 15, 1955, Serial No. 546,896

2 Claims. (Cl. 202-23) This invention relates to an improved apparatus for carrying out hydrocarbon oil fluid cokirig processes. More particularly the invention is concerned with an improved coking system wherein the coking reactor is provided with an annular stripping zone through which the solids circulate in the system and a separate oversized particle withdrawal means. v

There has recently been developed an improved process ltnown as the iiuid coking process for the production of lluid coke and the thermal conversion `of heavy hydrocarbon oils to lighter fractions, e. g. see allowed cases Serial No. 433,913, tiled June 2, 1954, now Patent No. 2,725,349, and Serial No. 431,412, tiled May V21, 1954, now Patent No. 2,721,169. For completeness, the process is described in further detail below although it should be understood that the lluid coking process itself is not the essence of this invention.

The fiuid coking unit consists basically of a reaction vessel `or Coker and a heater or burner vessel. In a typical operation the heavy oil to be processed is injected into the reaction vessel containing a dense turbulent iluidized bed of hot inert solid particles, preferably coke particles, but sand and spent catalyst can oe employed. Uniform temperature exists in the colting bed. Uniform mixing in the bed results in virtually isothermal conditions and effects instantaneous distribution of the feed stoclr. ln the reaction zone the feed stock is partially vaporized and partially cracked. Eliluent vapors are removed from the coking vessel and sent to a separation system, or the like for the recovery of gas and light distillates therefrom. Any heavy bottoms is usually returned to the coking vessel. The coke produced in the process remains in the bed coated on the solid particles. Stripping steam is injected into the stripper to remove oil from the colte particles prior to the passage of the coke to the burner.

The heat for carrying out the endothermic coking reaction is generated in the burner vessel, usually but not necessarily separate. A stream of coke is thus transferred from the reactor to the burner vessel, such as a transfer line or fluid bed burner, employing a standpipe and riser system; air can be supplied to the riser for conveying the solids to the burner. Suflicient colte or added carbonaceous matter is burned in the burning vessel to bring the solids therein up to a temperature sufficient to maintain the system in heat balance. The burner solids are maintained at a higher temperature than the solids in the reactor. About 5% of coke, based on the feed, is burned for this purpose. The net coke production, which represents the coke make less the coke burned, is withdrawn.

Heavy hydrocarbon oil feeds suitable for the coking process include heavy crudes, atmospheric and crude vacuum bottoms, pitch, asphalt, other heavy hydrocarbon petroleum residuals or mixtures thereof. Typically such feeds can have an initial boiling point of about 700 F. or higher, an A. P. I. gravity of about 0 to 20, and a Conradson carbon residue content of about 2 to so t Z,S39,45l Patented June 17, 1953 40 wt. percent. l(As to Conradson carbon residue see A. S. T. M. test D-189-4l.)

It is preferred to operate with solids having a particle size ranging between 100 and 1000 microns in diameter with a preferred particle size average diameter being between 100 and 400 microns. Preferably` not more than 5% has a particle size below about 75 microns, since small particles tend to agglomerato or are swept out of the system with the gases. While coke is the preferred particulate solid, other inert solids such as spent catalyst, pumice, sand, kieselguhr, Carborundum,I and alumina can be employed.

As the coking operation proceeds, the particles tend to grow rapidly by deposition thereon of coke formed in the process. If the particle growth during the pyrolysis is excessive and the particles reach sizes greatly above the cited limits, without being removed, erratic and interrupted ow occurs in the solids circulation system.

Furthermore, during the coking process coke deposits of appreciable size are apt to form on the reactor Walls and on other internal surfaces in the vessel. They eventually become dislodged and fall into the lluidized bed and nally find their way into the circulating solids system, causing diiculty.

Abnormal periods of coking operation also give rise to the formation of coke agglomerates. Removal of these agglomerates is essential to the smooth uninterrupted fluid coking process.

In addition improved stripping of the circulating coke is desirable in order to prevent carryover of liquid hydrocarbons to the coke burner.

According to the present invention means are provided for preventing oversized particles exceeding a predetermined size for proper fluidization from entering the particle circulation lines. ln addition improved stripping of the particles in the desired size range is accomplished. These objects are obtained in a coking reactor having a straight sided section communicating with a lower inverted conical section, the latter also communicating with a lower confined stripping section. A conduit for withdrawing oversized coke particles is placed centrally in radially spaced relationship in the stripper, thus defining an annular stripping zone therein. The conduit is perforated at its upper portion and terminates in an inverted conical section. The outer end of the conical section is attached to the upper end of the inner surface of the stripping section. This results in the withdrawal of predominantly all of the oversized particles through the conduit means and the segregating of predominately all, i. e. at least wt. percent, of the desired size particles in the annular stripping zone. These particles are stripped at low velocity in the annular zone and then circulated to the remainder of the system.

The withdrawal conduit is perforated at its upper end. These perforations extend down as needed to provide the desired free area but the perforations usually cover the conical section.

This invention will be better understood by reference to an example and the liow diagram shown in the drawlng.

In the drawing the vessel 1 is a coking vessel constructed of suitable materials for operation at 950 F. and contains a straight sided section 20, an inverted conical section 30 and a confined stripping section 40. A bed of coke particles preheated to a suliicient temperature, e. g. 1125 F., to establish the required bed temperature of 950 F. is made up of suitable particles of to 400 microns. The bed of solid particles reaches an upper level indicated by the numeral 5. The bed is fluidized by means of a gas such as steam at a temperature of 400 F. entering the vessel via pipes 3 and 15.

duced into the bed of hot coke particles via line 2,

preferably at a plurality of points in the system. The oil upon contacting the hot particles undergoes decomposition and the vapors resulting therefrom assist in the uidization of the solids in the bed and add to its general mobility and turbulent state. The product vapors pass upwardly through the bed and are removed from the coking vessel via line 4 after passing through cyclone 6 from which solids are returned to the bed via dipleg 7.

The solids in the bed move continually downward from the coker to the stripping section 40. Conduit withdrawal means 11 extends upwardly through the stripping section thus dening an annular stripping zone 12. The conduit terminates at its upper end in an inverted conical section 13, 30 from the Vertical. The conduit is Vperforated at its upper end from point 14. The perforations are of such size that particles in the desired range pass through into annular stripping zone 12, whereas the oversized particles larger than the desired range, e. g., larger than 400 microns, gravitate into the conduit withdrawal means 1K1. Inert stripping steam at slow velocities, e. g., 0.5 ft./sec. is injected into the annular zone through lines 15. Battles such as disc and doughnut type 41 are contained therein. Other gases such as light hydrocarbons and inert Hue gas can be used. The strippedV particles in the desired size range are withdrawn from the'annular stripping zone through line 16 to the burner not shown.

The conditions usually encountered in a uid coker for fuels are also listed below.

Pressure, Atmospheres Superficial Velocity of Fluidizing Gas, Ft./see Coke Circulation (Solids/Oil Ratio) Superficial Velocity of Stripping Gas, Fia/sec..

Conditions in burner Broad Range Preferred Range Temperature, F Superficial Velocity of Fluidizing Gas, Ft./

sec

The advantages of this invention will be apparent to those skilled in the art. Oversized particles capable of producing trouble are eliminated from the circulation lines Whereas improved stripping of the desired particles is efficiently obtained. Greater stripping volume is provided for a given unit height and yield losses due to carry'- over of oil to the burner are avoided.

It is to be understood that this invention is not limited to 'the speciiic examples which have been offered merely as illustrations and that modifications may be made without departing from the spirit of the invention.

What is claimed is:

l. In a process for coking a heavy hydrocarbon oil which comprises the steps of contacting the oil coking charge stock at a coking temperature with a body of inert particles having an average particle size in the range of to 400 microns maintained in the form of a dense, turbulent, iiuidized bed in a reaction zone having a lower conned stripping section, removing product vapors from the reactio-n zone, circulating the particles through an extraneous heating zone wherein a portion of the particles are heated and back to the reaction zone to supply heat thereto, and wherein oversized particles larger than specified accumulate and have to be removed, the improvement which comprises the steps of screening the oversized particles in an annular screening zone above an annular stripping zone; withdrawing the oversized particles from the stripping section through a coniined conducting zone countercurrent to injected iiuidizing gas for the dense bed, said Vconducting zone extending central of the stripping section and defining theannular stripping zone, sending predominately all the circulating particles from the coker into the annular stripping zone free of oversized particles, stripping them with an auxiliary inert gas at a superficial velocity lower than that of the fluidizing gas and circulating the thus stripped coke from the annular zone to the external heating zone.

2. The process of claim 1 in which the stripping gas has a superficial velocity of 0.5 to 1.5 ft./sec.

References Cited in the tile of this patent UNITED STATES PATENTS 

1. IN A PROCESS FOR COKING A HEAVY HYDROCARBON OIL WHICH COMPRISES THE STEPS OF CONTACTING THE OIL COKING CHARGE STOCK AT A COKING TEMPERATURE WITH A BODY OF INERT PARTICLES HAVING AN AVERAGE PARTICLE SIZE IN THE RANGE OF 100 TO 400 MICRONS MAINTAINED IN THE FORM OF A DENSE, TURBULENT, FLUIDIZED BED IN A REACTION ZONE HAVING A LOWER CONFINED STRIPPING SECTION, REMOVING PRODUCT VAPORS FROM THE REACTION ZONE, CIRCULATING THE PARTICLES THROUGH AN EXTANEOUS HEATING ZONE WHEREIN A PORTION OF THE PARTICLES ARE HEATED AND BACK TO THE REACTION ZONE TO SUPPLY HEAT THERETO, AND WHEREIN OVERSIZED PARTICLES LARGER THAN SPECIFIED ACCUMULATE AND HAVE TO BE REMOVED, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF SCREENING THE OVERSIZED PARTICLES IN AN ANNULAR SCREENING ZONE ABOVE AN ANNULAR STRIPPING ZONE, WITHDRAWING THE OVERSIZED PARTICLES FROM THE STRIPPING SECTION THROUGH A CONFINED CONDUCTING ZONE COUNTERCURRENT TO INJECTED FLUIDIZING GAS FOR THE DENSE BED, SAID CONDUCTING ZONE EXTENDING CENTRAL OF THE STRIPPING SECTION AND DEFINING THE ANNULAR STRIPPING ZONE, SENDING PREDOMINATLEY ALL THE CIRCULATING PARTICLES FROM THE COKER INTO THE ANNULAR STRIPPING ZONE FREE OF OVERSIZED PARTICLES, STRIPPING THEM WITH AN AUXILIARY INERT GAS AT A SUPERFICIAL VELOCITY LOWER THAN THAT OF THE FLUIDIZING GAS AND CIRCULATING THE THUS STRIPPED COKE FROM THE ANNULAR ZONE TO THE EXTERNAL HEATING ZONE. 